1. Field of the Invention
The present invention relates to a nitride-based semiconductor laser device, and more particularly, it relates to a nitride-based semiconductor laser device formed by successively crystal-growing an active layer, a cladding layer and the like.
2. Description of the Background Art
A nitride-based semiconductor laser device expected as the light source for an advanced large capacity optical disk has recently been actively developed. In general, a semiconductor laser device has important characteristics such as a threshold current and an operating current. The semiconductor laser device starts lasing with the threshold current, and the operating current is reduced as the threshold current is reduced. Therefore, the threshold current is preferably minimized.
Reduction of the threshold current is generally attempted also in the nitride-based semiconductor laser device. When the threshold current is reduced, the operating current necessary for driving the nitride-based semiconductor laser device can also be reduced and hence the device can be prevented from deterioration resulting from rise of the internal temperature following increase of the operating current. Thus, it is important to reduce the threshold current also for elongating the life of the nitride-based semiconductor laser device.
In order to reduce the threshold current, light spreading from an emission layer to a cladding layer must generally be reduced for efficiently confining light in the emission layer for the following reason: When the difference between the refractive indices of the emission layer and the cladding layer is reduced for increasing light spreading in a conventional nitride-based semiconductor laser device, the difference between the band gaps of these layers is so reduced as to increase the number of carriers (electrons and holes) overflowing from the emission layer into the cladding layer. If the number of the overflowing carriers is increased, it is difficult to emit light and hence the threshold current as well as the operating current are disadvantageously increased. In general, therefore, the difference between the refractive index of the emission layer and that of the cladding layer is increased to increase the difference between the band gaps thereof in order to inhibit the carriers from overflowing and reduce the threshold current. When the difference between the refractive indices of the emission layer and the cladding layer is increased, light spreading from the emission layer to the cladding layer is reduced and hence light is efficiently confined in the emission layer. Consequently, optical density is increased in the emission layer to increase the vertical beam divergence angle.
Thus, the conventional nitride-based semiconductor laser device is prepared to increase the vertical beam divergence angle to about 30°. For example, Japanese Journal of Applied Physics, Volume 39 (2000), L647-650 discloses such a nitride-based semiconductor laser device. The nitride-based semiconductor laser device disclosed in this literature has a vertical beam divergence angle of 29.9°.
However, the aforementioned conventional nitride-based semiconductor laser device having a large vertical beam divergence angle has the following problem: The nitride-based semiconductor laser device includes a larger number of crystal defects such as dislocations than an AlGaAs- or AlGaInP-based semiconductor laser device emitting infrared or red light. Further, the nitride-based semiconductor laser device emits purple or ultraviolet light having a short wavelength, and such short-wavelength light has large energy in lasing. The aforementioned conventional nitride-based semiconductor laser device having a large vertical beam divergence angle formed to reduce light spreading remarkably confines light in the emission layer. Therefore, the optical density is increased in the device to easily cause light absorption resulting from crystal defects such as dislocations. Such light absorption results in consumption of excess energy, to disadvantageously increase the operating current. When the operating current is increased, the internal temperature of the device rises to reduce the band gap, leading to large light absorption resulting from crystal defects. Consequently, the device is so abruptly deteriorated that it is difficult to elongate the life of the aforementioned nitride-based semiconductor laser device having a large vertical beam divergence angle.